Watch the video on Heating & Cooling System Sizing.
America likes to buy in bulk. If we can get products in the "jumbo", "extra large", or "mega" size, we generally feel we're getting a better value and more of what we want. But when it comes to a home's heating and cooling needs, super-sizing the mechanical system can cost the homeowner both money and comfort.
Over-sizing the HVAC system is a common mistake in new home construction. Over-sizing simply means that the heating, cooling, and fan units installed will deliver more conditioned air than the house requires. This can lead to higher initial costs at purchase, and higher operating and maintenance costs over the life of the house.
A system that is too powerful for the home may result in short cycle times. Because the system is able to deliver more conditioned air faster, the units turn on and off rapidly. This can mean rapid drastic changes in room temperature and can add to the wear and tear of equipment, reducing their overall lifespan. Short cycle times with air conditioning units also fail to dehumidify the living space properly, creating a damp-feeling living environment.
Proper sizing is critical to get the most comfort, greatest efficiency, and lowest overall maintenance from the heating and cooling system. The best practice for today's homebuilder is to insist that the heating and cooling trade contractor use correct sizing calculations and methods before hiring them to install the system.
The process for calculating the size of the HVAC system is tricky and requires careful study of the architectural drawings created for the home. These drawings are used to better understand how the house will be built. The heating and cooling loads are calculated using several variables:
- Surface areas of exterior walls, ceilings, floors, and foundations.
- The R-value of insulation in the home.
- The specific type of windows being used such as low-e or clear glazed.
- The load from the fresh air ventilation system.
- How airtight the ductwork is.
- How airtight the home is (if a builder doesn't know this, a blower door contractor can calculate this).
- Heat gain from lighting, appliances, and people should be considered for cooling loads only.
This data is fed into a computer program based upon calculations from Manual J, a manual for residential load calculations created by the Air Conditioning Contractors of America (ACCA) trade organization.
After taking into account the geographical information about the home (the historical extreme summer and winter temperatures experienced in that part of the country) the program analyzes all the data and reports two figures, the Total Heating Load, and the Total Cooling Load. These figures are measured in British Thermal Units per Hour or BTUH.
With these calculations, a mechanical engineer will be able to determine the required size and types of the home's three main environmental control units: the furnace, air-conditioning unit, and fan or air handling unit. It's important to install a fan that can adequately handle the load requirements of the system.
Though often well intentioned, over-sizing the HVAC systems can lead to higher costs and lower comfort for the home owner, as well as a shortened life-span for equipment. A better bet for the builder would be to pick the right system, and skip the super-size.
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